Hydrolytic potential of Trichoderma sp. strains evaluated by a microplate-based screening on switchgrass

Abstract

Introduction: Bioconversion of lignocellulosic biomass to fuel requires a hydrolysis step to obtain fermentable sugars, generally performed by fungal enzymes. The development of optimal enzyme cocktails for any specific feedstock would benefit from an assorted library of microbial strains. The aim of this study was to screen a large collection of Trichoderma strains for the hydrolytic potential when grown on a natural lignocellulosic material. Methods: More than 300 Trichoderma strains were cultivated stationary in flat bottom 24-well plates in an agarized medium containing milled switchgrass (Panicum virgatum L.) or microcrystalline pure cellulose (Galletti et al., 2008). Supernatants obtained by a rapid centrifugation step of thewhole culture plateswere evaluated in micro-plates for extracellular total cellulase and xylanase activities (Xiao et al., 2004). Results: The cellulase activity distributions for the entire Trichoderma collection showed similar profiles on both substrates (Fig. 1A) while a huge increase in xylanase activity median was observed on switchgrass compared to cellulose (Fig. 1B). A weak but positive correlation among individual cellulase and xylanase activities was noticed on both substrates (rcellulose = 0.6129; rswitchgrass = 0.6487), maybe due to a partially shared regulation pathway (Foreman et al., 2003). Strains originally isolated from lignocellulosic materials displayed a 4.8-fold increase in xylanase activity median on switchgrass with respect to cellulose, compared to a 3.1-fold increase in median for the strains isolated from other sources. A small group doubled cellulase activity when grown on cellulose instead of switchgrass, reaching the values displayed by the hyper-cellulolytic mutant T. reesei RutC30. Discussion: This study showed that the nature of the feedstock has a strong impact on the production of cellulase and xylanase by Trichoderma. The high variability observed in the screened collection could be exploited to develop strains producing feedstock-tailored enzymes.